Because of the extreme nature of the application, materials used for fabrication of ammunition cartridges must demonstrate excellent mechanical and thermal properties. The prevalent materials for production of cartridge casings for all calibers of ammunition in the world today are metals. Brass is the leading material, followed in smaller amounts by steel and, in limited amounts, aluminum. The use of polymeric materials for ammunition cartridge casings has been extensively investigated over the past 40 years, but success has been elusive.
Brass, steel, and, to a lesser degree, aluminum cartridge casings suffer from a number of disadvantages, the most important of which are heavy weight and corrosion concerns. Aluminum has an added disadvantage of potential explosive oxidative degradation and is thus used only in low-pressure cartridges or in applications that can tolerate relatively thick casing walls. Given these issues, desirable materials for ammunition cartridge casing fabrication would be lightweight and impervious to corrosion while having mechanical properties suitable for use in ammunition applications. Many lightweight polymeric materials are sufficiently corrosion resistant; however, to date, polymers have been used only in niche ammunition applications where their inferior mechanical and thermal properties can be tolerated (e.g., shotgun shells contain polyethylene components).
While stability under broad ranges of handling and storage conditions are crucial, the greatest mechanical demands on the cartridge material are experienced during the firing event. The material at the cartridge base end, which supports the primer, must first absorb the impact of a firing pin on the primer without mechanical failure. Upon ignition and combustion of an encapsulated propellant, rapidly expanding gases create high pressure, which expels a projectile from the barrel of the fired firearm. The ammunition cartridge casing must withstand and contain the pressure developed by the explosion so that the gaseous combustion products expand only in the direction of the barrel opening, thus maximizing energy conversion to projectile kinetic energy.
A firearm's cartridge chamber closely fits the outside of a cartridge and thus supports the majority of the cartridge casing wall in the radial direction; however, in many firearms, a portion of the cartridge base end protrudes from the chamber and is thus unsupported. During firing, a stress profile is developed along the cartridge casing, with the greatest stresses being concentrated at the base end. Therefore, the cartridge base end must posses the greatest mechanical strength, while a gradual decrease in material strength is acceptable axially along the casing toward the forward end which receives the projectile.
A typical brass cartridge casing is engineered to provide a strength profile along the casing length which reflects the varying mechanical demands, with the strongest and hardest material located at the cartridge base end. In brass and other metals, a strength profile is easily induced by varying the heat treatment conditions from one end of the casing to the other, but this is not an option for polymers. A mechanical strength profile can be achieved in a polymeric ammunition cartridge casing by varying the casing wall thickness; however, where the casing external geometry is fixed by existing firearm chamber size, an increased casing wall thickness often results in a casing with insufficient internal volume to accept the required propellant charge.
Many ammunition articles have been designed with cartridge cases comprised of two or more separate parts. The individual components are typically fabricated from different materials; a high-strength, usually metallic material comprises the cartridge casing base portion or “cap” while a polymer or other material comprises the remainder of the casing. For example, commercial shotgun ammunition employs a metallic base or cap joined to a polymeric top or sleeve. Although a significant amount of metal is required for such an ammunition cartridge, weight and cost savings can be sufficient to make it commercially acceptable.
While the most severe mechanical requirements of an ammunition cartridge are focused on the base end, the top or forward portion of the casing must meet several material requirements as well. Upon combustion of the cartridge propellant, a very large quantity of energy is released in a matter of a few milliseconds, thus producing very high stresses and strain rates. The casing material must possess adequate ductility to absorb the shock of the explosion without experiencing brittle fracture. Also, the material must possess sufficient rigidity and strength to avoid creep, flow, or other deformation.
A vast amount of effort has been dedicated to designing plastic ammunition cartridges, and researchers in the field have tested a variety of materials. Despite these attempts, consistent success has not been achieved.
Because of the demands on the casing material, the key problem in developing polymer-cased ammunition remains identifying a suitable polymeric material. It appears that all of the polymeric materials tried thus far are critically deficient in either their absorption of the impact energies generated during the firing event or their retention of mechanical integrity at high temperatures. A significant improvement in the art would be the identification of polymeric materials capable of at least serving as the top or forward portion of the ammunition cartridge casing.